First, car styling and aerodynamic resistance
Under atmospheric pressure, the average speed of the car, the aerodynamic resistance in horizontal driving is composed of differential pressure resistance and friction resistance, according to the proportion of the two and the different parts of the car, aerodynamic resistance is divided into shape resistance, induced resistance, roughness resistance and interference resistance, internal circulation resistance. While 69% of the fuel in the car is used to overcome air resistance, and 58% of the aerodynamic resistance is shape resistance, so the car modeling has a crucial impact on reducing its resistance. The wind resistance coefficient is introduced here
where υ is the relative velocity of the uniform air flow at infinity with the car, and A is the orthographic projection area of the car. Figure 1-1 is the aerodynamic drag coefficient when driving at a horizontal uniform speed in the same level of uniform ideal airflow of different models, and it can be seen that the aerodynamic drag coefficient of different models varies greatly.
Figure 1-1
Next, we will briefly analyze the above aerodynamic resistance one by one.
1
Shape resistance
80%-90% of the car's shape resistance is differential pressure resistance, and air friction resistance accounts for only 10%-20%. Fig. 1-2 is a demonstration of the differential pressure resistance of different bodies in the same ideal fluid:
Figure 1-2
Blunt bodies such as horizontally placed flat plates and cuboids increase the pressure at the maximum cross-section of the object, so that the fluid separates and a vortex is formed on the backflow side, which leads to an asymmetrical pressure distribution on the surface of the object, resulting in differential pressure resistance. Rounding allows flow around the edges to not produce separation. This allows the flow delay to separate and reduce the differential pressure resistance. Differential pressure resistance should also be analyzed in automobiles.
And for frictional resistance:
Figure 1-3
The speed gradient and molecular viscosity within the boundary layer at each point of the wall produce shear stress τ the sum of frictional resistance in the direction of flow.
When the flow does not produce separation, the frictional resistance accounts for a very large proportion of the total resistance, and vice versa, the proportion is very small. That's why golf can fly very far when it has a nest. But streamlined objects, such as some vehicles, have a large proportion of air friction resistance.
Because all liquids and gases have viscosity. This causes friction between adjacent fluid microgroups as they move at different speeds. Another consequence of viscosity is that fluid microcimers attach to the surface of the objects through which they flow. These attached fluid microcillections block the fluid microclutons flowing through them, creating frictional resistance.
Compared with the turbulent boundary layer, the laminar boundary layer has a weaker effect on the surface of the body, thereby reducing frictional resistance and energy absorption. Therefore, the frictional resistance of streamlined physics accounts for a relatively large proportion of the total resistance.
At this time, the surface of the car needs to be smoothed.
2
Induces resistance
Induced drag is due to different pressures on the lower surface of the body, but not lift. Due to the pressure difference between the surface and the lower surface of the body, a vertical flow component is superimposed on the horizontal incoming flow, bypassing the body side to balance the pressure, and the vortex that flows with the mainstream will be generated on the body side. The constantly generating vortex absorbs energy and thus induces resistance. Induced resistance is associated with pneumatic lift, which will be mentioned later.
Fig. 2-1 Schematic diagram of induced resistance of automobiles
3
Roughness resistance and interference resistance
Roughness resistance and interference resistance include all resistance caused by surface demarcation and attachments protruding from the body surface boundary layer. Chassis components and suspensions, wheels and mirrors, additional lights, wipers, etc. all contribute to roughness resistance and interference resistance.
Interference resistance is divided into positive interference resistance and negative interference resistance, as the name suggests, the effect of the two on resistance is opposite. Positive interference resistance is generated between two objects that are close to or connected. When the car is the main body, the rearview mirror will destroy the original flow field of the car, so that the air is separated prematurely and the aerodynamic resistance is increased. Negative interferometric drag refers to an area behind the fluid in each flow field where the flow rate decreases, and objects in this area are subjected to less resistance than outside. For example, two discs placed before and after (Fig. 2-2), at a certain distance, the sum of the resistance of the two is smaller than the resistance when placed separately. This is also reflected in the trailer: the aerodynamically optimised tow head effectively reduces the overall aerodynamic resistance.
Figure 2-2
4
Internal circulation resistance
Internal drag, or internal resistance, is the resistance produced when cooling engines and ventilation equipment to pass gas through the body of the car, including the loss of momentum of the fluid at the outlet and the loss of pressure of the fluid through the cooler and engine compartment, which results in additional energy loss. The internal circulation resistance accounts for about 5%-12% of the air resistance.
Second, car styling and aerodynamic lift
The aerodynamic lift of the car is derived from its special shape (Fig. 3-1).
Figure 3-1 According to the Bernoulli equation in the ideal state:
Combined with the shape of the car, ideally:
F <F
That is, there is aerodynamic lift.
In fact, taking into account the role of the car chassis, the simplified flow situation and pressure distribution of the non-sticky, sticky two-dimensional circumference of the car are shown in Figure 3-2.
Figure 3-2
The rolling resistance of the car is proportional to the normal force of the wheel, increasing the aerodynamic lift, then this normal force becomes smaller, so that the resistance is reduced. This seems to indicate that aerodynamic lift is beneficial to reduce the resistance of the car to drive. However, the increase in pneumatic lift will not only lead to a weakening of the drive and stability of the car, but also lead to additional induced resistance due to the increase in pneumatic lift, which may be much greater than the reduced rolling resistance. In this analysis, increasing pneumatic lift to reduce car resistance is not worth the loss, because a substantial increase in pneumatic lift will weaken the stability of the car driving, increase safety hazards, such as some cars above 70km/s will appear "drifting" this high-speed aerodynamic instability phenomenon, reduce the road sense, resulting in car rollover, drifting, etc.
The result was the Wedge, an aerodynamically optimised body that was even fitted with spoilers to increase downforce. Most of the wedge-shaped cars are sports cars, and their aerodynamic lift is even negative, which greatly increases the stability of the vehicle when moving at high speed.
(Some of the graphics in this article refer to the book Aerodynamics of road vehicle)
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Source: APC Science Alliance
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